Push-pull amplifier



Dec. 6, 1938.

H. BARTELS PUSHPULL AMPLIFIER Filed May 8, 1957 510/2 FACTOR-k v 1 a. 01 \1 INVENTOR HANS 5 QTELS BY movie/J ATTORNEY Patented Dec. 6, 1938 UNITED STATES 2,138,898 PATENT OFF'IQE Germany Application May 8,1937, Serial No. 141,442 In Germany February 28, 1936 4 Claims.

My present invention relates to push-pull amplifiers, and more particularly to push-pull amplifiers operating with constant grid bias potential. i

A tendency has been manifest for several years in the amplifier art to secure higher utilization of the tubes, and this has resulted in the development of the so-called class B amplifier. Inasmuch as the class B amplifier, for lower Signal amplitudes, has a higher blur or rattle factor (or non-linear harmonic distortion) than the A-type amplifier, amplifiers have been created in which the shortcoming of the B-type amplifier, namely, the said greater distortion at low amplitudes is avoided, while yet securing the merit inherent in better utilization or higher efficiency of the tubes. The tubes are adjusted to a quiescent current (that is, the current fiowing when no signal potential is applied to the grid) which amounts to about one-half the permissible plate dissipation in the case of A-type operation. Up to an alternating (signal) current whose crest, or peak, value is equal to the quiescent current, the tube operates like an A-class amplifier, while beyond that it works as a type B amplifier as a result of a shift of the grid potential as afunction of the amplitude. These amplifiers have come to be known as A prime or AB amplifiers.

However, the invention is concerned with an amplifier in which, just as inthe A-class amplifier known in the art, the operating point of the tubes is adjusted to the highest permissible plate dissipation, or practically to the said point, for all signal amplitude values and which, contradistinct to the said AB amplifier, operates with a a constant grid potential even in the presence of the highest amplitudes.

According to the invention, in an amplifier ad'- justed in its operating condition as stated, the terminal resistance is so proportioned that the tubes are operated dissymmetrically, or more pro cisely spealn'ng, in such a way that in each tube the loop or half-cycle of the plate current (when the tube is run under maximum load condition) corresponding to the positive grid voltage halfcycle, becomes twice as high as that of the negative grid voltage half-cycle or alternation. In this case, zero plate current will fiow in the (various) tubes even in the presence of median grid (signal) potentials.

What is here meant by dissymmetric driving operation is that, contradistinct to the A amplifier, under full-load condition, thefplate. current alternation corresponding to the positive grid potential alternation will turn out to be essentially larger, or, more particularly more than twice as large as with the negative grid potential loop or alternation. This feature has been known in the prior art as far as the applica'ntis aware only for amplifiers for which the working point was located at the bottom bend, or knee, of the mutual characteristic connecting the grid potential and the plate current and in which, therefore, the operating point, unlike the amplifier of the invention, is not adjusted to the highest permissible plate dissipation. As a matter of fact, in such an amplifier the utilization of the maximum permissible plate dissipation would not be feasible at all with modern tubes for the reason that the plate potential would have to be inadmissibly high to correspond to the low plate current. In other words, dissymmetric operation has never been used in the earlier art simultaneously with utilization of the highest permissible plate dissipation'.

' In carrying the basic idea of this invention into practice, the power or output is raised, while the non-linear distortion is made lower compared with the known Aclass amplifier for which identical tubes are employed.

An exact evaluation of the combined families of characteristics for push-pull circuit organizations has shown that, as a result of a reduction of the inner resistance of the tubes, in the presence of large plate currents, a considerable increase of power for the class A amplifier is feasible, if the amplifier where the invention is used, is operated with a substantially lower terminal impedance than what is nowadays found by customary calculation for modern single-grid type power tubes.

contrasted with the push-pull amplifier, class A, to be sure, the constituent tubes for higher powers, when considered separately are operated under markedly dissymmetric conditions, although actual measurements go to show that the blur factor (indicative of non-linear harmonic distortion), contradistinct to what has heretofore been generally believed,'will not rise correspondingly and will, in fact, not surpass a permissible limit. M

Selection of the optimal operating point and terminal impedance follows from, and is predicated upon, these conditions:

1. The increase in D. C. power (that is, the total power supplied by the plate current source) occurring upon overloading should not surpass the A. C. power supplied to the output circuit. The plate dissipation of the tube will then not exceed, in spite of appreciable overloading (which amounts to several times the quiescent plate current), the permissible value because the increase in D. C. power owing to the overloading is entirely utilized in supplying the increased A. C. power. In other words, contradistinct to both the class A and class B amplifiers, the plate dissipation can remain practically constant, also for different working or load points and amplitudes in fact, it is of maximum value even in the presence of vanishing grid alternating potential, 1. e., at the neutral or quiescent point.

2. The plate D. C. voltage, the grid bias potential, and the terminal impedance of the amplifier should be so chosen that the working characteristic, at low medium and high amplitudes, will be as straight as possible (with the highest possible power). These relations are easily ascertainable graphically by the aid of the combined family of characteristics. Such distortions as may arise may be further minimized by means of degeneration or anti-regeneration. Since as indicated, larger powers or outputs can be obtained from smaller tubes, it is necessary, with present day tubes, to drive the grid far into the positive range of the grid potentials whenever a particularly high power is to be secured. Nonlinear distortions which are occasioned by the grid current may then be lessened by auxiliary circuit means such as, for instance, by grid current feedback schemes known in the art.

In the drawing:

Fig. 1 shows characteristics used in the invention,

Fig. 2 graphically illustrates the advantage derived by using the invention.

By the aid of the diagram Fig. 1 the usual calculation method shall be explained in more detail. The abscissa gives the plate potentials and the ordinate the plate currents. There is also plotted the characteristic for the grid poten-. tial E =O as well as a positive grid potential characteristic. The working point is designated by P. This working point lies on the hyperbola plotted bythe dashed lines which indicates the plate dissipation A0, that is to say, the admissible tube load. Co-ordinated to this working point is a plate quiescent current I0 and a plate quiescent potential Eb. In the exemplified embodiment here shown a tube having 15 watts plate dissipation has been assumed. The resistance graph being rectilinear is denoted by Ra. In this example the plate potential swings :200 V. about the plate quiescent potential of 300 V. The inci: dental variations of current amount to approximately 45 milliamps. In other words, the plate potential in the present instance is utilized twothirds=66 percent, i. e.

An =10 Eb The terminal impedance is E, h E

I- 0 Transforming Equation (2) by the aid of Equation (1) there is obtained:

11- E a- A0 Inasmuch as factor it, as above indicated, is practically equal to from 0.6 to 0.8, there results:

E R (opt.)=0.6 to 0.8

Or upon the average Now, in the exemplified embodiment here chosen, E =300 V, Ao=15 watts, whence there follows a terminal impedance of 4200 ohms. If with a view to raising the output the range of positive grid potentials is included, then, as can be seen from the dot-dash working characteristics for Ra, Fig. 1, the optimal terminal impedance becomes higher; in other words, in Equation (1) the factors will be in the neighborhood of 0.8.

Now, contradistinct to the usual class A amplifier hereinbefore mentioned, the A-type amplifier of the invention, when used with present day single-grid tubes, operates with an essentially lower terminal impedance, to be more precise, an impedance amounting to only to the optimum impedance found from the calculation. The operation of this amplifier is indicated by the dashed straight-line resistance graph Rn which corresponds to a resistance of 1000 ohms. While it is true that the negative halves of the cycles are markedly cut by the abscissa axis in this mode of operation, it will be understood that, inasmuch as at that point the second tube of the push-pull arrangement begins to work, the said circumstance is unable to seriously impair conditions. The grid current distortions caused by driving into the positive range may be compensated by ways and means known in the prior art.

This amplifier distinguishes itself very favorably from the standard type of A amplifier by the fact that there is a substantial reduction of the terminal impedance combined with a simultaneous increase of the positive range of the grid potentials, contrary to what is true of present day dimension rules according to which resistance Ra with a view to preventing overloading must be chosen larger if the positive grid voltage range is included. (See straight dot-dash graph Ra, Fig. 1.) In this new working scheme, the plate D. C., even when the amplitudes are of but average value, will no longer stay constant, but will experience an essential rise so that, where large I amplitudes are involved, it may attain a value twice as high. The dimensions to be chosen for the output transformer and the input stage, and under certain circumstances also the selection of the working potentials differ essentially from the customary dimensions and values. But the new amplifier here disclosed agrees with the Class A amplifier of the prior art in so far as the maximum plate dissipation exists even when the grid (signal) alternating voltage is of zero value. However, while in the class A amplifier, the plate dissipation declines as the load or drive range of the tube grows, it stays practically stable and constant in the amplifier of the invention.

The distinction contrasted with the AB amplifier is that even in the presence of a maximum load or drive range, operation will never be purely or exactly that of the B-type amplifier. And the shift of the grid potential required in the AB amplifier as a function of the average value of the amplitude is inexistent.

What results in this mode of operation is an amplifier which shows only the low distortions of the A-type amplifier in a push-pull circuit organization, while yet being capable of delivering a far higher power. The drawbacks inherent in the B-class amplifier, i. e., more serious distortions at low amplitudes, and the shortcomings of the A B amplifier, namely, the occurrence of linear distortion of 5 percent, the output will be around 30 Watts which is three times higher, without the plate dissipation, which really governs the stress and strain of the tube, being exceeded. At an output of 10 watts, the non-linear distortion amounts to only 2 percent; and it is reduced still further for lower powers.

For this example, Fig. 2 shows the inter-relation between the non-linear distortion (blur factor) and the power output. Graph I refers to the usual A-class amplifier whose terminal impedance (referred to an individual tube) is 4200 ohms u while graph II illustrates the situation for the new A-type amplifier having a terminal impedance of about 1000 ohms (also referred to an individual tube or 500 ohms referred to one-half of or 2000 ohms referred tothe entire primary winding of the output transformer).

For two Telefunken tubes type REGGQ, using the dimension rule here disclosed, there is obtained a power of 20 watts approximately, while if the instructions of the prior art are followed, the output will range only between 5 and 6 watts. These findings have been confirmed experimentally; in both instances both the maximum ,plate dissipations as well as the plate D. C. potentials were the same.

Inasmuch as in the new operating method hereinbefore disclosed, the grid is driven far into the positive grid potential range, considerable grid currents will arise, and the grid resistance becomes comparatively low. This fact must be taken into consideration when choosing the inner resistance of the driver tube. Now, the following simple rule and relation may be laid down for the purpose of choosing these dimensions. Assuming that the grid current Ig-=0.1 Ia (where Ia=plate current) and that on the average the grid alternating potential is equal to A; the plate alternating potential, there results the following To prevent the voltage at the driver tube from collapsing during the positive half of the cycle, the inner resistance of the driver tube R1 driver must be substantially lower than the minimum grid resistance Rg min.. Tests have been shown that satisfactory results are obtainable in the presence of ratios less than 1:3, which means that 1 dl'iVBl' g min Now, since n (i:|1in.) a there will be 1 drivcr a In other words, the inner resistance of the driver tube must be lower than the load resistance of the power or end tube. The higher this ratio, the less will be the amount of distortion occasioned by the inner resistance of the driver tube.

Inasmuch as intermittent or jerky power delivery is here involved, the customary way of furnishing the grid bias potential by tapping a resistance included in the plate-filament circuit and thus traversed by plate current may here not be employed. In fact, a distinct grid voltage source of supply is required, or else additional or auxiliary means must be provided in order that a practically stable grid bias potential may be obtained even with a markedly fluctuating plate current. Also the plate potential source of supply must be suitably designed to suit the new operating conditions here disclosed. Inasmuch as plate currents of appreciable size are flowing, the inner resistance of the D. C. voltage source must be as low as feasible. Where amplifiers fed with A. C. are concerned, the said end is insurable by the use of a gaseous amplifier, or else by the insertion of stabilizing means of a kind known in the art, such as choke coils whose impedance varies as a function of the D. 0. load, or equivalent means.

What is claimed is:

1. Method of operating a push-pull amplifier which consists in adjusting the fixed bias relative to the plate voltage so that the non-signalling plate dissipation is the maximum permissible, and secondly adjusting the load resistance to the lowest value which does not cause the plate dissipation to increase appreciably in the presence of signal input up to the magnitude of input that causes the distortion to reach a predetermined value.

2. The method of increasing the maximum output of a push-pull vacuum tube amplifier which comprises choosing operating voltages such as to produce substantially the maximum allowable dissipation in the absence of signals and a zero signal current per tube sufficiently low so that the instantaneous current per tube may be increased by more than twice the zero signal current per tube, and choosing the load resistance sufficiently low so that at maximum signal input to the amplifier the instantaneous current per tube increases by more than twice the zero signal current per tube, whereby each tube is driven to substantially the maximum allowable average dissipation in the presence of maximum input to the amplifier as well as during the zero signal condition.

3. The method of operating a push-pull amplifier including a pair of tubes having an output load resistance and fixed energizing supply voltages which comprises choosing said voltages to produce maximum allowable dissipation in the absence of signal input, and choosing said load resistance to cause substantially maximum allowable dissipation in the presence of maximum signal input.

4. The method of operating a push-pull vacuum tube amplifier having fixed operating voltages and a resistance load which comprises choosing said voltages to bring the operating point of said amplifier on the linear portion of its characteristic but below the middle portion thereof, and also to cause maximum allowable dissipation in said amplifier in the absence of signal input, whereby on weak signals the amplifier operates class A with a dissipation that decreases with increasing signal input for a range of signal inputs, and also choosing said load resistance so low that on strong signals the dissipation again increases and approaches said maximum allowable value.

HANS BARTELS. 

